The transgenesis of MEO, that can be e.g. a plant or an animal, comprises the integration of a transcription unit into the MEO's genome that comprises a gene and a regulatory sequence that controls the transcription of said gene. To achieve regulation over a maximum amount of orders of magnitude, it is favorable to employ as a regulatory sequence one or more transcription control sequence(s) susceptible for binding transactivators fused to an enhancerless promotor. The basal activity of a chromosomally integrated minimal (i.e. enhancerless) promotor strongly depends on its chromosomal locus. It is well known that the activity of a transcription unit (consisting of at least a promoter, a gene to be expressed and a polyadenylation site), which is inserted into a chromosomal locus either by random or targeted integration, depends strongly on the context of the surrounding chromatin (Palmiter et al. Annu. Rev. Genet. 29, 465-99). When inserted into heterochromatin, transcription units are usually silenced. In euchromatin, on the other hand, the activity of an inserted transcription unit depends on a number of parameters which may affect transcription of the integrated gene positively or negatively. When the transcriptional unit is located close to an enhancer, it will show an increased activity even in the absence of its cognate activator. This problem can be circumvented, if non-homologous recombination is used, by increasing the number of clones (i.e. different integration events) to be screened, or more actively by shielding the transcriptional unit via transcriptional silencers which may be susceptible to control by ligands. However, it is obvious that the first solution of the problem apart from being time consuming leads to the binding of personal and financial resources. The latter solution of the problem is complicated and often reduces the maximum level of transcription of the gene that can be achieved. Furthermore, also the maximum level of transcription of the gene that can be achieved also varies with the chromosomal locus of integration. The locus may also give rise to position effect variegation (“PEV”, Palmiter et al. Annu. Rev. Genet. 29, 465-99) and thus, to mosaic expression of the transgene.